When confluent monolayers of WI-38 human diploid fibroblasts are stimulated to proliferate by fresh medium containing 10% serum, as many as 80% of the cells enter DNA synthesis after a lag period of about 12 to 15 hours. In the first hour after stimulation, the chromatin of the stimulated WI-38 cells undergoes a number of functional and structural changes. These functional and structural differences between chromatins of stimulated and unstimulated cells are abolished when both chromatins are washed with 0.25 M NaCl, a procedure that removes about 10% of total chromosomal proteins. The present application proposes to investigate the component of chromatin that is responsible for these functional and structural changes, and to determine its nature and origin. In addition since a structural difference has also been demonstrated between the chromatins of WI-38 cells and their SV-40 transformed counterparts, 2RA cells, the present proposal intends to investigate a component of chromatin that is responsible for such a difference and to relate it to the changes found in quiescent cells stimulated to proliferate. The significance of this research lies in the fact that with these studies it would be possible to obtain valuable information on the components of chromatin that play an important role in the regulation of normal and abnormal cell growth.